In general, in wireless communication devices such as cellular phones, a method for reducing the power consumption of a transmission-side power amplifier by controlling the power used at the time of transmission may be used in a radio frequency (RF) circuit. As an example of such a method, there is a method in which a directional coupler is connected between a transmission-side power amplifier and an antenna, power is detected by extracting part of a transmission signal output from the transmission-side power amplifier by using the directional coupler and the detected power is fed back to a control unit that controls the transmission-side power amplifier so that the magnitude of the power remains constant.
FIG. 1 illustrates the configuration of an antenna device described in Japanese Unexamined Patent Application Publication No. 5-291854 (Patent Document 1). As illustrated in FIG. 1, the antenna device described in Patent Document 1 includes a transmission-side power amplifier 201, a directional coupler 202, an impedance converter 203, a detector circuit 204, an error voltage amplifier 205 and a bias circuit 206. The directional coupler 202 is connected between the transmission-side power amplifier 201 and an antenna. In the antenna device described in Patent Document 1, a transmission signal, which is an input RF signal, is input to the transmission-side power amplifier 201 and is then amplified and output by the transmission-side power amplifier 201. At this time, part of the transmission signal output from the transmission-side power amplifier 201 is extracted by the directional coupler 202. The transmission signal extracted by the directional coupler 202 has a bias voltage superposed thereon by the bias circuit 206 and is then input to the impedance converter 203. The bias voltage superposed by the bias circuit 206 is for correcting the forward voltage drop of a diode of the detector circuit 204. The impedance converter 203 is configured so that its output impedance is larger than its input impedance and therefore the voltage of a signal output from the impedance converter 203 is higher than the voltage of a signal input to the impedance converter 203. A signal output from the impedance converter 203 is detected by the detector circuit 204. The signal output from the detector circuit 204 is input to the error voltage amplifier 205 together with a signal output from a level control voltage generator based on data corresponding to an instructed power level from a base station stored in a ROM. The error voltage amplifier 205 amplifies and outputs a difference between the two signals input thereto and a signal output from the error voltage amplifier 205 is fed back to a control unit that controls the amplification ratio of the transmission-side power amplifier 201.